1. Field of the Invention
The present invention relates to a line printer for the digital output and colorimetric measurement of colored images.
2. Description of Prior Art
Color printers and line printers, respectively, are widely used in the form of digital output devices forming part of a computer system for processing colored images. Computer systems of this type are used, in particular, also in the DTP area (Desk Top Publishing area) for designing pictures and printed pages that, subsequently, are multiplied in large numbers on a printing press. DTP systems of this type comprise a computer having a color screen, a scanner for the digitalized input of the data of the images to be processed in the computer, a film exposing means for the output of films for subsequently copying the same on printing plates, and a digital color output device. The color output device provides a print-out of the images processed by the DTP system, or of printed pages, with the colors of the digital print-out corresponding to the original picture or to the desired result on the printing press. Digital color output devices generate, as a rule, the printable color volume by over-printing or side-by-side printing of several process colors (usually cyan, magenta, yellow and black). The color output device is provided by the computer with digital control data providing information on the ratio at which the individual process color channels of the printing head are to be actuated. There is no generally applicable relationship between the said control data and the color actually generated on the output medium, because the properties of the material to be printed and the process colors, as well as parameters such as ambient temperature and humidity, will determine the actually resultant color. Different output devices, processes and media, with identical control data, result in different color output. The underlying object, therefore, resides in identifying the color characteristics of the output device. A mathematical relationship is established between the color control data and the colors actually generated in the output process. This process is designated by color characterization or color profiling. The result is a so-called color profile.
Presently, the color characterization is in three steps. During the first step, a so-called color test chart is provided by the color output device. The said test chart comprises a number of color fields. Each individual color field is generated by defined color control data. The color control data for the fields is so selected as to cover the maximum color gamut of the output device. The color control data representing the test chart are sent by a computer to the color output device.
During the second step, each individual field of the test chart is subjected to a calorimetric measurement by a color measuring device which forms hand-held measuring instrument manually positioned above each individual color field, or XY-coordinate measuring tables on which the test chart is positioned, with a motor driven measuring instrument operating to expose color field by color field to calorimetric measurements, with the resultant data being transmitted to a computer.
During the third step, the calorimetric data, by a color profiling software, is mathematically related to the color control data to compute a color profile. Specific color discrepancies of an output device can thus be compensated by employing a specific color profile.
The test charts used for color characterization purposes comprise a large number of differently colored measuring fields. To insure complete characterization of the entire color volume, the test charts contain between 400 and 2000 color fields. For example, the standardized test chart ANSI IT 8.7/3- 1993 has 928 color fields sized 6.times.6 mm. Measuring the large number of color fields in succession with the aid of a hand-held measuring instrument would be time-consuming and troublesome even if the measured values after each individual measurement were automatically transmitted, via a data cable, by the measuring instrument to the computer computing the color profile after completion of the measurements. Moreover, the monotonous repetition of the measuring operation would easily result in operating errors. The evaluation on a coordinate measuring instrument would require additional working space and would be complex to perform with especially high technical efforts involved.